Program session group to route to next available session agent if non-responsive

Hello Group,

Looking for some assistance.  I'm working on programming a session group with multiple members where at time these session agents (modems) show as active but when you send a call the agent does not respond after the 180 ringing message.  This is an issue at the distant end but need to configure the sbc that if it encounters an INVITE that never receives an ACK message, to route the call to the next available agent in that group.